Integrated circuit with reduced substrate coupling

ABSTRACT

The integrated circuit includes a substrate SB incorporating a plurality of electronic components C 1 , C 2  and a seal ring SR around the electronic components. It includes cold spot means VM, PG, BDG disposed between the electronic components and the seal ring. It further includes electrostatic discharge protection means including an electrostatic discharge rail VM around the electronic components and constituting said cold spot means.

[0001] The invention relates to integrated circuits and moreparticularly to radio-frequency integrated circuits designed to operateat frequencies of 1 GHz and above.

[0002] At the present time, radio-frequency integrated circuits includevarious functional units within the same substrate, for examplelow-noise amplifiers (LNA), local oscillators, power amplifiers andphase-locked loops.

[0003] One of the major problems to be solved in designingradio-frequency circuits is to minimize the risk of coupling between thevarious units. This is because any coupling from one unit to another cancause a malfunction of the integrated circuit, as in the case ofcoupling between a low-noise amplifier of a receive channel and a poweramplifier of a send channel, for example. This is because coupling isthen created between the input and the output of the circuit, whichcauses a malfunction of the circuit.

[0004] There are currently many causes of coupling between units. Forexample, coupling between units can result from interconnection problemsor from design errors in the layout of the electronic components.Coupling can also occur via the silicon substrate of the integratedcircuit.

[0005] The silicon substrate is inherently conductive. The variousgrounds of the integrated circuits are therefore connected via theresistance of the substrate.

[0006] To minimize, or even eliminate, coupling between units via thesubstrate, it has already been proposed to place insulative walls in thesubstrate between the various functional units.

[0007] However, there is another problem, previously unidentified, andwhich is also the cause of coupling between the various functional unitsof an integrated circuit via the substrate.

[0008] This problem lies in the presence of a seal ring around theelectronic components of the integrated circuit, which is inherent tothe technology.

[0009] The seal ring prevents pollution of the integrated circuit bypolluting particles resulting from cutting the semiconductor wafer alongcutting paths delimiting the various integrated circuits of the wafer.

[0010] The seal ring is in fact a wall made up of all the layersavailable in the integrated circuit as well as a caisson formed in thesubstrate, for example a P⁺-type caisson.

[0011] Because of its nature, and because it has a higher conductivity,the seal ring constitutes a low-impedance path. Consequently, if aninterfering signal is emitted into the substrate by a functional unitlocated near the seal ring, that interference signal will naturally takethe low-impedance path consisting of the seal ring and propagate alongthe ring, possibly until it creates unwanted coupling with anotherfunctional unit in another part of the integrated circuit and alsobeside the seal ring.

[0012] The invention aims in particular to provide a solution to thisnew problem, most especially for radio-frequency integrated circuits.

[0013] In this regard, the invention proposes providing the integratedcircuit with means forming a cold spot between the electronic componentsof the integrated circuit and the seal ring.

[0014] As the skilled person is well aware, cold spot means consist of apath of minimum impedance, enabling unwanted signals or charges to beevacuated via the cold spot means, and consequently preventing unwantedsignals or charges flowing in the seal ring to interfere with theoperation of another functional unit of the integrated circuit.

[0015] Another constraint in the manufacture of integrated circuits isthe need to provide them with means to protect them againstelectrostatic discharges that could otherwise damage the electroniccomponents of the integrated circuit.

[0016] The means providing protection against electrostatic dischargesconventionally consist of a plurality of discharge rails (metallization)connected to the respective functional units and in particular havingcommon ground terminals.

[0017] These means also include devices formed by diodes and bipolartransistors for circulating electrostatic charges from one terminal ofthe integrated circuit to another, preventing them passing through thevarious functional units.

[0018] In one particularly advantageous embodiment of the invention, andsubject to a structural modification, the electrostatic dischargeprotection means constitute said cold spot means. To be more precise, inaccordance with the invention, the electrostatic discharge protectionmeans include at least one electrostatic discharge rail surrounding theelectronic components and forming the cold spot means.

[0019] In one embodiment of the invention, the electrostatic protectionrail is connected to an external ground terminal via connecting meansand the impedance of the rail and the connecting means is lower thanthat of the seal ring. Also, the discharge rail has no direct connectionto the bulk substrate of any component.

[0020] The invention therefore adopts a “monorail” strategy toelectrostatic discharge protection, the “monorail” strategy solving theproblem of coupling via the substrate and the seal ring by forming thecold spot means.

[0021] Because in some configurations or types of package, theconnecting means consist of electrical wires (bonding wires) havingnon-negligible inductance, the connecting means advantageously include aplurality of electrical wires (bonding wires), for example at least fourand advantageously about ten, in particular if the BGA, TQFP or QFN formof package is used.

[0022] The invention also proposes a method of minimizing the couplingvia the substrate between various electronic components of aradio-frequency integrated circuit, including the installation of a coldspot means between the electronic components and a seal ring surroundingthe electronic components.

[0023] The cold spot means are advantageously produced with anelectrostatic discharge rail surrounding the electronic components.

[0024] According to a mode of implementation in which at least some ofthe electronic components have bulk substrates, the electrostaticprotection rail is connected to an external ground terminal byconnecting means. The impedance of the rail and the connecting means isless than that of the seal ring, and the discharge rail is made to haveno direct connection with the bulk substrate of any component.

[0025] The invention also proposes a use of a cold spot means disposedbetween the electronic components of a radio-frequency integratedcircuit and a seal ring surrounding the electronic components, so as tominimize the coupling via the substrate between these various electroniccomponents.

[0026] Other advantages and features of the invention will becomeapparent on reading the following detailed description of onenon-limiting embodiment and from the accompanying drawings, in which:

[0027]FIG. 1 is a highly diagrammatic illustration of one embodiment ofan integrated circuit according to the invention;

[0028]FIG. 2 is a diagram showing a seal ring more specifically and fromanother point of view; and

[0029]FIG. 3 is a highly diagrammatic illustration of one effect of theinvention.

[0030]FIGS. 1 and 2 show a radio-frequency integrated circuit CI afterit has been cut from a semiconductor wafer by cutting along cuttinglines.

[0031] The integrated circuit is formed in a substrate SB, for example aP-type substrate. It has at its periphery a seal ring SR. As shown inFIG. 2 in particular, which shows the integrated circuit partly insection, the seal ring conventionally comprises a P⁺-type caisson and astack of all the layers of the integrated circuit.

[0032] The seal ring therefore forms a wall around the electroniccomponents of the integrated circuit to prevent any pollution whencutting the integrated circuit CI away from the semiconductor wafer.

[0033] For simplicity, it is assumed here that the integrated circuitincludes electronic components forming two separate functional units(cores) C1 and C2. For example, the unit C1 can provide a low-noiseamplifier (LNA) function and the unit C2 a power amplifier function.

[0034] As is conventional in the art, and as the skilled person is wellaware, it is particularly advantageous to provide between the functionalunits C1 and C2 an isolating wall for mutually isolating the twofunctional units from unwanted signals that could pass from onefunctional unit to another via the substrate SB. This kind of isolatingwall is formed by a P⁺-doped caisson, for example, connected to ground.

[0035] Unwanted signals can also propagate in the substrate from theunit C1, for example, toward the periphery of the integrated circuit,i.e. toward the seal ring SR.

[0036] The general feature of the invention then consists of disposingbetween the electronic components and the seal ring SR cold spot means,i.e. means for collecting unwanted signals and evacuating them to anexternal ground, in order to prevent any coupling with the otherelectronic components, for example the unit C2. With no such cold spotmeans, unwanted signals from the unit C1, for example, and propagatingtoward the seal ring would then flow in the seal ring, which constitutesa low-impedance path, in particular because of the presence of the P⁺caisson. The unwanted signal would then propagate via the seal ring andcould reach the bulk substrate of the components of the functional unitC2, with the risk of causing malfunctions.

[0037] Here the cold spot means advantageously take the form of anelectrostatic discharge rail VM around the electronic components andforming part of electrostatic discharge protection means.

[0038] The skilled person knows that an integrated circuit is usuallyprovided with protection means against electrostatic discharges that canoccur between two leads of the package, for example if the package ispicked up by hand or by a placement machine.

[0039] The electrostatic discharge protection means usually include thesame number of rails as there are functional units in the integratedcircuit (this is known as a “multirail” strategy). Also, the groundterminals of the functional unit are common to the ground terminal ofthe corresponding electrostatic discharge rail.

[0040] The invention proposes a totally different electrostaticdischarge protection strategy which provides efficient protectionagainst electrostatic discharge and additionally provides the cold spotmeans.

[0041] In this regard, the electrostatic discharge protection strategyis a “monorail” strategy, that is to say a strategy using a singleelectrostatic discharge rail VM connected to an external ground terminalby connecting means, the discharge rail having no direct connection tothe bulk substrate of any component.

[0042] In addition to the rail VM, the electrostatic dischargeprotection means include another rail VP parallel to the rail VM andwhich also surrounds the electronic components of the integratedcircuit. The rail VP is not connected to ground.

[0043] The connections by electrical connecting wires (“bonding wires”)between the terminals of certain components and the leads of the packageencapsulating the integrated circuit are all connected to the rail VMvia the diodes D1 and to the rail VP via the diodes D2. To be moreprecise, the anode of a diode D1 is connected to the rail VM and thecathode of a diode D2 is connected to the rail VP.

[0044] The electrostatic discharge protection means further include acircuit CLM known in the art and conventionally comprising diodes andbipolar transistors, for example, for detecting electrostatic dischargepulses and enabling them to pass from one rail to the other. The circuitCLM is known to the skilled person as a “clamp”.

[0045] For simplicity, the example shown in FIG. 1 includes a singleelectronic component in the unit C1, for example a bipolar transistor T.The base of the bipolar transistor is connected to the terminal PB andto a metal pin PTB of the package by a connection (bonding wire) BD. Thedrain of the transistor is connected to a terminal PD in turn connectedto a pin PTD of the package via a wire BD.

[0046] The source of the transistor T is connected to a terminal PS inturn connected to a pin PTS of the package via a wire BD. Finally, thebulk substrate BK of the transistor is also connected to the terminalPS.

[0047] If an electrostatic discharge occurs between the pin PTB and thepin PTS, for example, during handling of the package containing theintegrated circuits, and assuming that this discharge takes the form ofa positive pulse, it travels from the terminal PB toward the rail VP viathe diode D2, and then follows the circuit CLM, traveling along the railVM and leaving via the pin PTS, the diode D1 and the terminal PS.

[0048] A negative pulse takes the rail VM from the terminal PB, via thediode D1, and then the circuit CLM, the rail VP, and the diode D2,before it leaves via the terminal PS.

[0049] Of course, the skilled person knows that the diodes D1 and D2constitute paths of minimum impedance relative to the connectionsbetween the terminals and the component itself. The connectionsincorporate resistances (for example the base resistance or the sourceresistance) which have higher impedances. It is also conventional todispose other diodes (not shown here) between the components and theterminals PB and PS, for example, to guarantee that the diodes D1 and D2constitute a minimum impedance path. This is well known to the skilledperson.

[0050] For the rail VM to form a cold spot, the impedance of the railand the connecting means that connect it to an external ground terminalmust be lower than the impedance of the seal ring SR. The skilled personknows that the connecting wires (bonding wires) connecting the terminalsof the integrated circuit to the external metal pins of the package havean inductive impedance. For this reason it is preferable to have aplurality of terminals PG connected to the rail VM, with each terminalconnected by an electrical connecting wire BDG to a pin PTM of thepackage; all the pins PTM can be connected to an external ground whenthe integrated circuit is operating.

[0051] By providing several parallel BDG connecting wires, it ispossible to minimize, or even eliminate, the unwanted inductanceinherent to the connecting wires, which is particularly important forradio-frequency applications, thus making it possible to provide a coldspot means in terms of radio-frequency.

[0052] The skilled person knows how to adjust the number of terminals PGnecessary to achieve a cold spot (path of minimum impedance). A numberof the order of ten is chosen, for example, for thin quad flat packs(TQFP).

[0053] Thus in operation, if an unwanted signal flows from the unit C1via the substrate toward the seal ring, it takes the preferred pathconsisting of the rail VM and is evacuated to ground via the pins PTM.

[0054] Note that, unlike the prior art, the electrostatic discharge railVM has no direct connection to the bulk substrate of any electroniccomponent of the integrated circuit. This is because the terminal PS ishere connected to the rail VM via the diode D1. Accordingly, thedistance between the source of an unwanted signal (the transistor T, forexample) and the seal ring SR is greatly increased relative to a priorart structure. This is because, in the prior art, because there is adirect connection between the bulk substrate BK and the terminal PSwhich constitutes both the ground terminal of the transistor and theground terminal of the rail, an unwanted pulse emitted into thesubstrate by the transistor T is directly at the level of the terminalPS, and therefore very close to the seal rail.

[0055]FIG. 3 shows the advantages of the invention. FIG. 3 shows a pathCH1 that can be taken by an unwanted signal emanating from eitherfunctional unit C1 or C2. It can be seen that this path is interruptedby the isolating wall MI. However, in the prior art, an unwanted signalcould equally short circuit the isolating wall and travel via the sealring SR, causing unwanted coupling via the substrate.

[0056] In accordance with the invention, on the other hand, the coldspot means between the functional units C1 and C2 and the seal ring SRform a minimum impedance path CH2 that evacuates an unwanted signal toground instead of causing it to travel through the seal ring SR. Thiseliminates the unwanted coupling.

1. An integrated circuit, including a substrate (SB) incorporating aplurality of electronic components (C1, C2) and a seal ring (SR) aroundthe electronic components, characterized in that the integrated circuitis a radio-frequency circuit and includes cold spot means (VM, PG, BDG)disposed between the electronic components and the seal ring.
 2. Anintegrated circuit according to claim 1, characterized in that itfurther includes electrostatic discharge protection means including anelectrostatic discharge rail (VM) around the electronic components andconstituting said cold spot means.
 3. An integrated circuit according toclaim 2, wherein at least some of the electronic components have bulksubstrates (BK), characterized in that the electrostatic protection rail(VM) is connected to an external ground terminal by connecting means(BDG), in that the impedance of the rail and the connecting means isless than that of the seal ring (SR), and in that the discharge rail(VM) has no direct connection with the bulk substrate of any component.4. An integrated circuit according to claim 3, characterized in that theconnecting means include a plurality of electrical wires (BDG).
 5. Amethod of minimizing the coupling via the substrate between variouselectronic components of a radio-frequency integrated circuit, includingthe installation of a cold spot means (VM, PG, BDG) between theelectronic components and a seal ring surrounding the electroniccomponents.
 6. A method according to claim 5, characterized in that thecold spot means are produced with an electrostatic discharge rail (VM)surrounding the electronic components.
 7. A method according to claim 6,wherein at least some of the electronic components have bulk substrates(BK), characterized in that the electrostatic protection rail (VM) isconnected to an external ground terminal by connecting means (BDG), inthat the impedance of the rail and the connecting means is less thanthat of the seal ring (SR), and in that the discharge rail (VM) is madeto have no direct connection with the bulk substrate of any component.8. A method according to claim 7, characterized in that the connectingmeans is formed with a plurality of electrical wires (BDG).
 9. The useof a cold spot means (VM, PG, BDG) disposed between electroniccomponents of a radio-frequency integrated circuit and a seal ringsurrounding the electronic components, so as to minimize the couplingvia the substrate between these various electronic components.